The plate convergence and oceanic expansion triggered by the aggregation and cleavage of Pangea have been the frontier hotspots of global plate tectonics research since the late Paleozoic. Located in central and eastern Asia, the Central Asian orogenic belt (also known as the Altai orogenic belt) is the ancient Asian Ocean tectonic domain, which records the collision and stitching process of various blocks of Eurasia caused by the closure of the ancient Asian Ocean, and is an important orogenic belt to construct the current Eurasian pattern. At the same time, the Central Asian orogenic belt is one of the most significant areas of the global Phanerozoic crustal proliferation and transformation, and it is also the world's largest continental mineralization area, and its mineralization process is closely related to the growth and orogeny, tectonic system transformation and Cenozoic transformation.
The timing of the final controversy of the southern margin of the Central Asian orogenic belt and the formation of Pangea has been the focus of international academic debate (Triassic, Permian or Late Carboniferous?). ), which has led to considerable uncertainty about the tectonic pattern of the northeastern margin of Pangea and the reconstruction of the ancient Asian continent (Proto-Asia). Despite the work of predecessors on this scientific issue, it has not been possible to achieve a unified understanding. At the same time, the long history of hypertrophic collage, the huge scale of spreading, and the superposition and transformation of huge magma/metamorphism/complex tectonic deformation in the Central Asian orogenic belt also increase the difficulty of clarifying this scientific problem. From the opening and expansion of the ancient Asian Ocean to the subduction and subduction of the oceanic crust, until the closure of the ocean basin and orogeny, sedimentation runs through it. Although the stratigraphic sequence has been fragmented by the alteration of tectonic movements, the strata (and the fossil record) remain partially preserved. The study of tectonic evolution is inseparable from the determination of geological time, and sedimentary tectonics has always played and will continue to play an important role in the study of the evolution of the ancient Asian Ocean.
The team of Xiao Wenjiao, academician of the Chinese Academy of Sciences and researcher of the Xinjiang Mineral Resources Research Center of the Xinjiang Institute of Ecology and Geography of the Chinese Academy of Sciences, selected a pre-arc hyperplasia basin located in the Akyazi-ultra-high pressure metamorphic hybrid belt of the southern Tianshan orogenic belt (Figure 1) to carry out research on the chronology of clastic zircon and the evolution of sedimentary geotectonics, and combined with the geochemical and chronological big data summary of the three major rock types in the region, the sources of sedimentary debris in the process of closing the South Tianshan Ocean in the southwest margin of the ancient Asian Ocean were systematically sorted out. Spatial-temporal evolution between shell-mantle responses and regional geodynamic lattices.
The results show that the main clastic source of the pre-arc growth basin is not from the Kazakh-Ili-Zhongtianshan continent in the north or the Karakum-Tarim craton in the south, but may be derived from the Triassic arc magma of the eastern Tiantian Mountain (Fig. 2a, e). Therefore, the main detritus source in the basin may have been transported by the Contour current from the more distant Eastern Tien Shan Arc (Figure 1e), which could limit the Southern Tien Shan Ocean closure time limit on the southwestern margin of the Palaeasian Ocean. Middle Permian MOR type snake greenstones (eastern Tiantian Mountain glacial plate) and late Carboniferous and Late Permian radiolaria fossils (Kyrgyzstan and the Southern Tien Shan region within the territory) further hint at the possibility of a younger ocean basin (Figure 2f). The basin may have extended into the early Triassic, and the isobaths were able to carry detrital material from the more distant East Tian Shan arc to form the pre-arc growth basin in the Aqyazi ultra-high pressure metamorphic hybrid zone seen today (Figure 3). This corroborates the Triassic cryothermal chronology cooling "signal" that is widely distributed regionally and associated with the eventual stitching of the southern margin of the Central Asian orogenic belt (Figure 2). Combined with reports in the Triassic hyperplasia basins of the South Tianshan, East Tianshan, central Inner Mongolia and Solonkel suture belt, the study shows that the entire ancient Asian Ocean closed in the western, central and eastern parts of the Central Asian orogenic belt or near the same period. This further indicates significant differences in the spatial and temporal distribution of subduction-related magma in the western and eastern regions of the South Tian Shan orogenic belt (Figure 3). Among them, due to the continuous subduction of the Devonian-Carboniferous seamount chain and the existence of cratonic microland masses in the west, the formation of slab subduction structures and their trapping of the subarc mantle wedge were formed, and their magmatic action basically stopped during 300-230 million years. Eastern arc magmatic action persisted from 330 million years until basin closure.
The research was published in Communications Earth & Environment under the title Final closure of the Paleo Asian Ocean basin in the early Triassic. The research work was supported by the National Natural Science Foundation of China and the major science and technology projects of Xinjiang Uygur Autonomous Region.
Fig. 1.Tectonic background and sample characteristics of pre-arc hyperplasia basin in the Aktaz metamorphic hybrid belt of the southern margin of the Central Asian orogenic belt.
Figure 2.Summary of chronology and geochemical big data of the South Tianshan orogenic belt.
Figure 3.Schematic diagram of the Permian-Triassic tectonic model of the Paleo-Southern Tianshan Ocean (STO). NTC: North Tarim Craton, AC: Prearc Proliferative Hybrid, KYCTC: Kazakhstan-Ili-Zhongtianshan Continent.
Source: Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences
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